CARRIER SYSTEMS ON TELEPHONE LINES 589 



Crosstalk. Telephone circuits which are simultaneously operating in 

 close proximity on a pole line are normally subject to crosstalk because 

 of the mutual inductance and capacity relations between the wires. 

 The problem which this presents in a pole line structure carrying 

 many circuits requires careful consideration, even where the fre- 

 quencies are no higher than the voice range. The problem is cared 

 for by the application of transposition systems, i.e., arrangements 

 whereby the effect of these relations between the circuits tends to be 

 canceled out by transposing the wires constituting the two sides of a 

 circuit in an orderly fashion. These transposition systems are care- 

 fully designed and the transpositions to be applied in each circuit 

 specified.^ 



When using still higher frequencies for carrier purposes, this problem 

 is correspondingly increased as the mutual relations tend to become 

 greater at higher frequencies. The phase changes as the currents 

 progress along the lines are more rapid for the higher frequencies. 

 The design of the transposition system capable of permitting the 

 simultaneous operation of a number of carrier systems on the same 

 pole line is a difficult problem. The subject is one of great complexity 

 and to give it complete consideration would require more space than is 

 available here. It may be noted, however, that, by means of special 

 transposition layouts installed in the circuits being used for carrier 

 transmission, successful operation is being obtained with a large 

 number of carrier systems on the same pole line, both telephone and 

 telegraph. The locations of transpositions in circuits used for carrier 

 transmission occur more frequently than in circuits restricted to 

 operation at voice frequencies, in some cases as frequently as every 

 other pole. 



Several factors in the apparatus design have contributed to lessen 

 the hardship imposed by the crosstalk problem: 



1. The standardization of arrangements whereby the same frequencies 



are only employed in a given direction on systems on the same 

 pole line. 



2. The equalization of the transmission levels between paralleling 



systems. 



3. The use of "staggered" frequency allocations for systems in 



closest proximity. 



4. A careful consideration of impedance relations in the line circuits 



and apparatus. 



^"The Design of Transpositions for Parallel Power and Telephone Circuits," 

 H. S. Osborne, A. I. E. E. Transactions, V. 37, June 1918, pp. 897-936. 



